The present invention relates to a pharmaceutically compatible method for purifying intact bacterial minicells.
A minicell is an anucleate form of an E. coli or other bacterial cell, engendered by a disturbance in the coordination, during binary fission, of cell division with DNA segregation. Prokaryotic chromosomal replication is linked to normal binary fission, which involves mid-cell septum formation. In E. coli, for example, mutation of min genes, such as minCD, can remove the inhibition of septum formation at the cell poles during cell division, resulting in production of a normal daughter cell and an anulceate minicell (de Boer et al., 1992; Raskin & de Boer, 1999; Hu & Lutkenhaus, 1999; Harry, 2001).
In addition to min operon mutations, anucleate minicells also are generated following a range of other genetic rearrangements or mutations that affect septum formation, for example in the divIVB1 in B. subtilis (Reeve and Cornett, 1975; Levin et al., 1992). Minicells also can be formed following a perturbation in the levels of gene expression of proteins involved in cell division/chromosome segregation. For example, overexpression of minE leads to polar division and production of minicells. Similarly, chromosome-less minicells may result from defects in chromosome segregation, for example the smc mutation in Bacillus subtilis (Britton et al., 1998) spoOJ deletion in B. subtilis (Ireton et al., 1994), mukB mutation in E. coli (Hiraga et al., 1989), and parC mutation in E. coli (Stewart and D'Ari, 1992). Gene products may be supplied in trans. When over-expressed from a high-copy number plasmid, for example, CafA may enhance the rate of cell division and/or inhibit chromosome partitioning after replication (Okada et al., 1994), resulting in formation of chained cells and anucleate minicells (Wachi et al., 1989; Okada et al., 1993).
Minicells are distinct from other small vesicles that are generated and released spontaneously in certain situations and, in contrast to minicells, are not due to specific genetic rearrangements or episomal gene expression. Exemplary of such other vesicles are bacterial blebs, which are small membrane vesicles (Dorward et al., 1989). Blebs have been observed in several bacterial species from Agrobacterium, Bacillus, Bordetella, Escherichia, Neisseria, Pseudomonas, Salmonella and Shigella, for example. Bacterial blebs can be produced, for instance, through manipulation of the growth environment (Katsui et al., 1982) and through the use of exogenous membrane-destabilizing agents (Matsuzaki et al., 1997).
Because plasmid replication within prokaryotic cells is independent of chromosomal replication, plasmids can segregate into both normal daughter cells and minicells during the aberrant cell division described above. Thus, minicells derived from recombinant min E. coli carry significant numbers of plasmid copies, with all of the bacterial cellular components except for chromosomes, and have been used as such in studying plasmid-encoded gene expression in vitro. See Brahmbhatt (1987), Harlow et al. (1995), and Kihara et al. (1996). Brahmbhatt (1987) demonstrated, for example, that E. coli minicells can carry recombinant plasmids with DNA inserts as large as 20 kb, absent any chromosomal DNA, and can express nine or more recombinant proteins simultaneously.
A recent patent application, PCT/IB02/04632 (incorporated entirely herein by reference), described recombinant, intact minicells containing therapeutic nucleic acid molecules. Such minicells are effective vectors for delivering oligonucleotides and polynucleotides to host cells in vitro and in vivo. Accordingly, they are particularly useful for introducing nucleic acid molecules that, upon transcription and/or translation, function to ameliorate or otherwise treat a disease or modify a trait associated with a particular cell type, tissue or organ of a subject.
In vivo minicell applications generally require minicell preparations of a high purity, particularly with respect to live parent bacteria, free endotoxin and cellular debris (including membrane fragments, nucleic acids and intracellular components) that might elicit an inflammatory response in an immunized host. Moreover, the use of minicells in commercial pharmaceutical products will require methods for purifying minicells to approved international pharmaceutical standards. To this end, conventional methods of minicell purification generally are unsatisfactory.
Conventional techniques entail (a) low speed centrifugation, to reduce the bio-burden of parent cells, and (b) differential rate sedimentation in a gradient of glycerol, sucrose or percoll. An initial differential, low speed centrifugation typically reduces parental cells by as much as 100-fold, while leaving 50% to 70% of minicells in the supernatant fluid. Two subsequent cycles of differential rate sedimentation then yield minicell preparations having a purity of about 1 vegetative cell per 106-107 minicells. Such conventional methods are reviewed by Frazer & Curtiss (1975), and are described by Reeve (1979), Clark-Curtiss & Curtiss (1983), and U.S. Pat. No. 4,311,797 (to Khachatourians).
The purity achieved by conventional purification methods may not be adequate for an in vivo applications, some of which may require doses greater than 106 minicells, or even 1010 minicells. At the aforementioned contamination ratio, this would translate into 10,000 live parent cells per dose. Such a contamination level could be fatal, particularly in immuno-compromised patients such as cancer and AIDS patients. For example, the ID50 (infectious dose in 50% of infected people) for Shigella dysenteriae, Salmonella enteritidis and Listeria monocytogenes organisms is approximately 10, 1,000 and 10 respectively. Moreover, previous studies have reported that the level of cell contamination varies with different bacterial strains (Clarke-Curtiss and Curtiss, 1983). In that regard, gene therapy applications described in PCT/IB02/04632 may employ minicells derived from a range of mutant Gram negative and Gram-positive bacterial strains, and would require minicells that are essentially free of live parent bacterial ceil contamination. Thus, conventional minicell purification methods do not permit quality control for cGMP (current good manufacturing practice) manufacture of biopharmaceutical doses of minicells.
As an additional drawback, the gradient formation media (percoll, sucrose and glycerol) employed by conventional purification methods are incompatible with in vivo uses. Percoll is toxic and, hence, is restricted to “research purposes only” contexts. Sucrose imparts a high osmolarity to gradients that can cause physiological changes in minicells. Indeed, the present inventors have determined that minicells undergo an osmotic shock in sucrose gradients and, as a consequence, become structurally deformed. Glycerol is highly viscous and difficult to remove completely from the minicell suspensions. Accordingly, although these density gradient media effectively separate cells and cellular organelles or components, they are not suitable for separating biological cells that are destined for clinical use in humans.
Several approaches have been developed to improve conventional minicell purification techniques. One approach employs parent cells that carry a chromosomal recA mutation, and treatment with low doses of Ultra Violet (UV) radiation (Sancar et al., 1979). The rationale of this approach is that UV radiation will preferentially degrade chromosomal DNA because of its large target size, as opposed to smaller plasmid DNA. However, recombinant minicells used for gene therapy and vaccine applications must be free of any mutation, and non-specific mutagenesis methods such as UV radiation would not ensure that all plasmid DNAs remain un-mutated.
Another approach to improve minicell purification operates by inhibiting bacterial cell wall synthesis, such as by using ampicillin or cycloserine, or by starving diaminopimelic acid (DAP)-requiring strains of DAP (Clarke-Curtiss and Curtiss, 1983). This approach also suffers from several drawbacks, however. First, many recombinant plasmids used for gene therapy will carry an ampicillin resistance marker, which renders parent cells carrying the plasmid ampicillin resistant. Second, many in-vivo minicell applications will employ minicells derived from a range of different bacterial species, many of which may not be susceptible to DAP-requiring mutations. Third, any large-scale use of antibiotics is undesirable due to the attendant risks of generating antibiotic-resistant bacteria.
Recently, a novel. approach for purifying minicells that addresses the above-mentioned concerns was reported (PCT/IB02/04632). The novel method combines cross-flow filtration (feed flow is parallel to a membrane surface; Forbes, 1987) and dead-end filtration (feed flow is perpendicular to the membrane surface) to achieve a minicell purity that exceeds 10−7 (i.e., fewer than one parent cell per 107 minicells), and even 10−9. Optionally, the filtration combination can be preceded by a differential centrifugation, at low centrifugal force, to remove some portion of the bacterial cells and thereby enrich the supernatant for minicells.
Although this filtration procedure overcomes the drawbacks associated with conventional minicell purification techniques, it also has limitations. Foremost, cross-flow filtration results in considerable loss of minicells, which adds cost to the manufacturing process. Additionally, minicell preparations obtained by the filtration procedure contain some bacterial endotoxin, which causes a mild shock when administered in vivo. Finally, minicell purity varies from batch to batch when the filtration methods are employed.
Therefore, a need remains for methods of purifying bacterial minicells that maximize minicell yield and purity, while employing biologically compatible media.
To address these and other needs, the present invention provides a method for purifying bacterial minicells that involves subjecting a sample containing minicells to density gradient centrifugation in a biologically compatible medium. The method optionally includes a preliminary differential centrifugation step.
The present invention also provides a method for purifying bacterial minicells that combines density gradient centrifugation in a biologically compatible medium with filtration.
In another aspect, the present invention provides a minicell purification method in which a sample containing minicells is subjected to a condition that induces parent bacterial cells to adopt a filamentous form, followed by filtration of the sample to separate minicells from parent bacterial cells.
In yet another aspect, the present invention provides a minicell purification method that includes (a) subjecting a sample containing minicells to density gradient centrifugation in a biologically compatible medium, (b) subjecting the sample to a condition that induces parent bacterial cells to adopt a filamentous form, then (c) filtering the sample to obtain a purified minicell preparation.
The inventive methods optionally include one or more steps to remove endotoxin from purified minicell preparations, and/or treatment of purified minicell preparations with an antibiotic.
Finally, the present invention provides purified minicell preparations, prepared according to the foregoing methods, and containing fewer than about 1 contaminating parent bacterial cell per 107, 108, 109, 1010 or 1011 minicells.